After a well has been drilled, it must be completed before it can produce gas or oil. Once completed, a variety of events may occur to the formation causing the well and its equipment to require a “work-over.” For purposes of this application, “work-over” and “service” operations are used in their very broadest sense to refer to any and all activities performed on or for a well to repair or rehabilitate the well, and also includes activities to shut in or cap the well. Generally, work-over operations include such things as replacing worn or damaged parts (e.g., a pump, sucker rods, tubing, and packer glands), applying secondary or tertiary recovery techniques, such as chemical or hot oil treatments, cementing the wellbore, and logging the wellbore, to name just a few. Service operations are usually performed by or involve a mobile work-over or well service rig (collectively hereinafter “service rig” or “rig”) that is adapted to, among other things, pull the well tubing or rods and also to run the tubing or rods back in to the well. Typically, these mobile service rigs are motor vehicle-based and have an extendible, jack-up derrick complete with draw works and block.
During rod or tubing removal, a rig operator typically lifts a stand of tubing (or rods) which is then held in place by slips (or elevators for rods) while the stand is separated from the remaining portion of the tubing or rod string in the well. Once the stand of tubing has been separated from that which is still in the well, the stand of tubing can be placed on a tubing board. During the initial lifting operation, the weight or load on the hook can fluctuate greatly based on the weight of the tubing string in the well, the conditions within the well, the condition of the tubing string, and the amount of acceleration of the tubing string. In general the tubing string acts similarly to a rubber band. As the operator begins to accelerate the block upward and pull the tubing string out of the well. the tubing string initially becomes elongated for a short interval before the entire tubing string begins to move upward through the well. The same elongation can occur when a portion of the tubing string encounters a part of the well with increased friction or gets snagged or stuck within a portion of the well. If the operator does not recognize the problem quickly enough, the amount of load on the hook (“hookload”) can increase very quickly to a level that is above the safe operating level of the rig. While alarms can be employed, if the operator cannot act quickly enough, the rig may be damaged and workers around the well could be injured.
In addition, as the stands of tubing (or rods) are being pulled out of the well, the total amount of weight on the string is reduced and the length of the string is reduced. When there are only a few stands of tubing left in the well, pulling the tubing out at a typical rate of speed, for example, six feet per second, can become more dangerous because if the tubing snags or drags in the well there is less overall elasticity within the remaining length of tubing, and therefore, less time to react to the increase in hookload. This can cause dangerous conditions around the wellhead.
Furthermore, while a stand of tubing (or rods) is being decoupled from the remaining string in the well, the operator brings his engine RPM up to drive the tongs that are used to unscrew the tubing from one another. When the previously pulled stand of tubing is fully disengaged from the remaining tubing in the well, the operator engages the clutch for the hoist and lifts the stand of tubing about another foot or two and places it onto the tubing board. The lifting of the stand of tubing that small distance prior to placement on the tubing board can cause a small spike in the rig load recorded at the rig load sensors. Much of this spike is caused by the acceleration of the block by the operator. Unfortunately, at times, the operator is in a hurry or is not cautious enough and can begin lifting the stand of tubing before the stand has been fully unscrewed from the tubing that remains in the well. When this occurs the rig load will suddenly and violently increase. The rig load can continue to increase until the stand of tubing breaks free of the final threads of the tubing at the wellhead. When the stand breaks free anyone in the vicinity of the wellhead is in danger of serious injury.
What is needed is a method and apparatus for evaluating the rig load or hookload of a service rig when removing tubing or rods from a well and disengaging the clutch for the hoist when the rig load reaches a level indicative of a problem with the tubing in the well, such as a snag or hang up. Furthermore, what is needed is a method and apparatus for evaluating the rig load or hookload of a tubing or rod string being removed from a well and limiting the speed of the block and hoist when only a small amount of tubing or rods remains in the well. In addition, what is needed is a method and apparatus for determining when a stand of tubing or rods is being decoupled from tubing or rods remaining in the well during a pull operation and preventing or limiting the ability for the block and hoist to lift the stand if the stand is not fully disengaged from the remaining tubing or rods in the well.
The present invention is directed to solving these as well as other similar issues in the well service area.